In today's high density magnetic recording art, the number of tracks per inch (TPI) has been increasing rapidly. To avoid erasure of adjacent tracks during data writing and to shield the main writing pole fringing fields, a side shield (SS) was added. Recently, a wrap-around shield (WAS) writer comprising a trailing shield together with side shields, as shown in FIG. 1 [1˜3] has been extensively studied, and is utilized in products to enable the areal recording density of hard disk drives to continue to grow.
There is, however, a problem associated with the side shield and WAS designs: wide area track erasure (WATE). When the writer is writing the data track, some percentage of heads can erase data several tracks away, usually after several cycles of write operation. WATE can occur from 1 to 10 μm away from the main writing pole location.
FIG. 2 shows some characteristics of WATE: the degraded bit error rate (BER) as a function of offset position to the main pole of 10 heads (each line is one head) was measured. The y-axis is the amount of BER degradation after the recording head has written on the central track for a certain amount of cycles. The x-axis is the offset from the center of the write pole. The write pole's (track) magnetic width is around 0.1 μm. It is obvious that some heads show strong erasure features at 1.2-1.5 μm away from the main pole. This far-track erasure phenomenon is detrimental to the disk drive reliability since the data not intended to be erased at those positions (1.2-1.5 μm away from the central track) can be accidentally erased. Testing for far-track WATE prior to drive-build is economically prohibitive so pre-screening is not an option. Solutions must therefore be found that eliminate these WATE peaks. Some of the root causes of WATE have been discovered [1-3], but many remain unknown at present.    1) Daniel Z. Bai, et. al. “High Density Perpendicular Recording with Wrap-Around Shielded Writer”, TMRC 2009, Paper B4    2) M. Mallary et. al, “One terabit per square inch perpendicular recording conceptual design”, IEEE Trans. Magn., vol. 38, pp. 1719-1724, July 2002.    3) S. Li, et. al. “Side track erasure processes in perpendicular recording”, IEEE Trans. Magn., vol. 42, pp. 3874-3879, December 2006.
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Pat. No. 7,538,976, Hsiao et al. teach a tapered trailing shield to prevent wide angle track erasure while in U.S. 2007/0230045, Hsiao et al. disclose recessed shield portions to prevent WATE. Guan et al. (Headway) show shields having recessed edges to avoid concentration of flux at the edges in U.S. Pat. No. 7,599,152. Okada et al. describe recessed shields to prevent leaking of the magnetic field in U.S. 2003/0026039. In U.S. 2009/0262464, Gill et al. disclose a wrap-around shield made of low-permeability material to reduce WATE while in U.S. 2007/0268623 Feng teaches a multi-layer pole structure to reduce WATE.